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Topological magnetoelectric response in ferromagnetic axion insulators

National Science Review 2024年第2期11卷 8-14页

作者： Yuhao Wan Jiayu Li Qihang Liu Department of Physics and Shenzhen Institute for Quantum Science and Engineering(SIQSE) Southern University of Science and Technology Shenzhen Key Laboratory of Advanced Quantum Functional Materials and Devices Southern University of Science and Technology Guangdong Provincial Key Laboratory for Computational Science and Material Design Southern University of Science and Technology

The topological magnetoelectric effect （TME） is a hallmark response of the topological field theory,which provides a paradigm shift in the study of emergent topological ***,its direct observation is yet to be realized due to the demanding magnetic configuration required to gap all surface ***,we theoretically propose that axion insulators with a simple ferromagnetic configuration,such as the MnBi2Te4/（Bi2Te3）nfamily,provide an ideal playground to realize the *** the designed triangular prism geometry,all the surface states are magnetically *** a vertical electric field,the surface Hall currents give rise to a nearly half-quantized orbital moment,accompanied by a gapless chiral hinge mode circulating in ***,the orbital magnetization from the two topological origins can be easily distinguished by reversing the electric *** work paves the way for direct observation of the TME in realistic axion-insulator materials.

关键词： topological magnetoelectric effect axion insulator magnetic topological materials

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quantum spin Hall effect protected by spin U(1) quasisymmetry

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Physical Review B 2024年第16期110卷 L161104页

作者： Lu Liu Yuntian Liu Jiayu Li Hua Wu Qihang Liu Department of Physics and Shenzhen Institute for Quantum Science and Engineering (SIQSE) Laboratory for Computational Physical Sciences (MOE) State Key Laboratory of Surface Physics and Department of Physics Shanghai Qi Zhi Institute Shanghai 200232 China Nanjing 210093 China Guangdong Provincial Key Laboratory of Computational Science and Material Design

quantum spin Hall (QSH) insulators, with their unique helical edge states where counterpropagating edge channels possess opposite spins, have attracted broad interest across various fields. While the exact quantization of spin Hall conductance (SHC) is elusive in realistic materials due to intrinsic spin mixing effects, the near quantization, as a compromised definition of the QSH effect, cannot be captured by rigorous topological invariants. In this Letter, we present a universal symmetry indicator for diagnosing the QSH effect in realistic materials, termed spin U(1) quasisymmetry. Such a symmetry eliminates the first-order spin-mixing perturbation and thus protects the near-quantization of SHC, applicable to time-reversal-preserved cases with either Z2=1 or Z2=0, as well as time-reversal-broken scenarios. We propose that spin U(1) quasisymmetry is hidden in the subspace spanned by the doublets with unquenched orbital momentum and emerges when SOC is present, which can be realized in 19 crystallographic point groups. Our theory is applied to identify previously overlooked QSH phases such as time-reversal-preserved even spin Chern phase and time-reversal-broken phase, as exemplified by twisted bilayer transition metal dichalcogenides, monolayer RuBr3, and monolayer FeSe. Our work provides a comprehensive symmetry-based framework for understanding the QSH effect and significantly expands the material pool for the screening of exemplary material candidates.

关键词： Electronic structure First-principles calculations Symmetry protected topological states Topological materials Dichalcogenides Two-dimensional electron system Z_2 symmetry

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Layer Hall effect induced by hidden Berry curvature in antiferromagnetic insulators

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National Science Review 2024年第2期11卷 15-23页

作者： Rui Chen Hai-Peng Sun Mingqiang Gu Chun-Bo Hua Qihang Liu Hai-Zhou Lu X.C.Xie Shenzhen Institute for Quantum Science and Engineering and Department of Physics Southern University of Science and Technology(SUSTech) International Quantum Academy Institute for Theoretical Physics and Astrophysics University of Würzburg School of Electronic and Information Engineering Hubei University of Science and Technology Guangdong Provincial Key Laboratory of Computational Science and Material Design Southern University of Science and Technology Shenzhen Key Laboratory of Quantum Science and Engineering International Center for Quantum Materials School of PhysicsPeking University Collaborative Innovation Center of Quantum Matter CAS Center for Excellence in Topological Quantum Computation University of Chinese Academy of Sciences

The layer Hall effect describes electrons spontaneously deflected to opposite sides at different layers, which has been experimentally reported in the MnBi2Te4thin films under perpendicular electric fields. Here, we reveal a universal origin of the layer Hall effect in terms of the so-called hidden Berry curvature, as well as material design principles. Hence, it gives rise to zero Berry curvature in momentum space but non-zero layer-locked hidden Berry curvature in real space. We show that, compared to that of a trivial insulator, the layer Hall effect is significantly enhanced in antiferromagnetic topological insulators. Our universal picture provides a paradigm for revealing the hidden physics as a result of the interplay between the global and local symmetries, and can be generalized in various scenarios.

关键词： layerHall effect hidden Berry curvature antiferromagnetic insulator axion insulator, quantum transport

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Group Theory on Quasisymmetry and Protected Near Degeneracy

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Physical Review Letters 2024年第2期133卷 026402-026402页

作者： Jiayu Li Ao Zhang Yuntian Liu Qihang Liu Department of Physics and Shenzhen Institute for Quantum Science and Engineering (SIQSE) Guangdong Provincial Key Laboratory for Computational Science and Material Design Shenzhen Key Laboratory of Advanced Quantum Functional Materials and Devices

In solid state systems, group representation theory is powerful in characterizing the behavior of quasiparticles, notably the energy degeneracy. While conventional group theory is effective in answering yes-or-no questions related to symmetry breaking, its application to determining the magnitude of energy splitting resulting from symmetry lowering is limited. Here, we propose a theory on quasisymmetry and near degeneracy, thereby expanding the applicability of group theory to address questions regarding large-or-small energy splitting. Defined within the degenerate subspace of an unperturbed Hamiltonian, quasisymmetries form an enlarged symmetry group eliminating the first-order splitting. This framework ensures that the magnitude of splitting arises as a second-order effect of symmetry-lowering perturbations, such as external fields and spin-orbit coupling. We systematically tabulate the quasisymmetry groups within 32 crystallographic point groups and find all the possible unitary quasisymmetry group structures regarding double degeneracy. Applying our theory to the realistic material AgLa, we predict a “quasi-Dirac semimetal” phase characterized by two tiny-gap band anticrossings.

关键词： Electronic structure Topological materials Density functional theory Group theory

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Rational design of large anomalous Nernst effect in Dirac semimetals

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npj computational materials 2023年第1期9卷 258-264页

作者： Panshuo Wang Zongxiang Hu Xiaosong Wu Qihang Liu Shenzhen Institute for Quantum Science and Engineering and Department of Physics Southern University of Science and Technology518055 ShenzhenChina State Key Laboratory for Artificial Microstructure and Mesoscopic Physics Frontiers Science Center for Nano-optoelectronicsand Collaborative Innovation Center of Quantum MatterPeking University100871 BeijingChina Guangdong Provincial Key Laboratory of Computational Science and Material Design Southern University of Science and Technology518055 ShenzhenChina Shenzhen Key Laboratory of Advanced Quantum Functional Materials and Devices Southern University of Science and Technology518055 ShenzhenChina

Anomalous Nernst effect generates a transverse voltage perpendicular to the temperature *** has several advantages compared with the longitudinal thermoelectricity for energy conversion,such as decoupling of electronic and thermal transports,higher flexibility,and simpler lateral ***,a design principle beyond specific materials systems for obtaining a large anomalous Nernst conductivity(ANC)is still *** this work,we theoretically demonstrate that a pair of Dirac nodes under a Zeeman field manifests an odd-distributed,double-peak anomalous Hall conductivity curve with respect to the chemical potential and a compensated carrier feature,leading to an enhanced ANC compared with that of a simple Weyl semimetal with two Weyl *** on first-principles calculations,we then provide two Dirac semimetal candidates,i.e.,Na3Bi and NaTeAu,and show that under a Zeeman field,they exhibit a sizable ANC value of 0.4 Am^(-1)K^(-1) and 1.3 Am^(-1)K^(-1),respectively,near the Fermi *** an approach is also applicable to ferromagnetic materials with intrinsic Zeeman splitting,as exemplified by a hypothetical alloy NaFeTe2Au2,exhibiting an ANC as high as 3.7 Am^(-1)K^(-1) at the Fermi *** work provides a design principle with a prototype band structure for enhanced ANC pinning at the Fermi level,shedding light on the inverse design of other specific functional materials based on electronic structure.

关键词： structure Nernst Dirac

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Enumeration and Representation Theory of Spin Space Groups

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Physical Review X 2024年第3期14卷 031038页

作者： Xiaobing Chen Jun Ren Yanzhou Zhu Yutong Yu Ao Zhang Pengfei Liu Jiayu Li Yuntian Liu Caiheng Li Qihang Liu Department of Physics and Shenzhen Institute for Quantum Science and Engineering National Center for Applied Mathematics Shenzhen and Department of Mathematics Guangdong Provincial Key Laboratory of Computational Science and Material Design Shenzhen Key Laboratory of Advanced Quantum Functional Materials and Devices

Fundamental physical properties, such as phase transitions, electronic structures, and spin excitations, in all magnetic ordered materials, were ultimately believed to rely on the symmetry theory of magnetic space groups. Recently, it has come to light that a more comprehensive group, known as the spin space group (SSG), which combines separate spin and spatial operations, is necessary to fully characterize the geometry and underlying properties of magnetic ordered materials. However, the basic theory of SSG has seldom been developed. In this work, we present a systematic study of the enumeration and the representation theory of the SSG. Starting from the 230 crystallographic space groups and finite translation groups with a maximum order of eight, we establish an extensive collection of over 100 000 SSGs under a four-index nomenclature as well as international notation. We then identify inequivalent SSGs specifically applicable to collinear, coplanar, and noncoplanar magnetic configurations. To facilitate the identification of the SSG, we develop an online program that can determine the SSG symmetries of any magnetic ordered crystal. Moreover, we derive the irreducible corepresentations of the little group in momentum space within the SSG framework. Finally, we illustrate the SSG symmetries and physical effects beyond the framework of magnetic space groups through several representative material examples, including a candidate altermagnet RuO2, spiral spin polarization in the coplanar antiferromagnet CeAuAl3, and geometric Hall effect in the noncoplanar antiferromagnet CoNb3S6. Our work advances the field of group theory in describing magnetic ordered materials, opening up avenues for deeper comprehension and further exploration of emergent phenomena in magnetic materials.

关键词： Electronic structure Magnetic order Spin texture Noncollinear magnets Density functional theory Group theory

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Polaron interfacial entropy as a route to high thermoelectric performance in DAE-doped PEDOT:PSS films

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National Science Review 2024年第3期11卷 259-270页

作者： Jiajia Zhang Caichao Ye Genwang Wei Liang Guo Yuhang Cai Zhi Li Xinzhi Wu Fangyi Sun Qikai Li Yupeng Wang Huan Li Yuchen Li Shuaihua Wang Wei Xu Xuefeng Guo Wenqing Zhang Weishu Liu Department of Materials Science and Engineering Southern University of Science and Technology Academy for Advanced Interdisciplinary Studies and Guangdong Provincial Key Laboratory of Computational Science and Material Design Southern University of Science and Technology Department of Mechanical and Energy Engineering Southern University of Science and Technology Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences College of Chemistry and Molecular Engineering National Biomedical Imaging Centre Peking University Guangdong Provincial Key Laboratory of Functional Oxide Materials and Devices Southern University of Science and Technology

Enhancing the thermoelectric transport properties of conductive polymer materials has been a long-term challenge, in spite of the success seen with molecular doping strategies. However, the strong coupling between the thermopower and the electrical conductivity limits thermoelectric performance. Here, we use polaron interfacial occupied entropy engineering to break through this intercoupling for a PEDOT:PSS(poly（3,4-ethylenedioxythiophene）-poly（4-styrenesulfonate）) thin film by using photochromic diarylethene（DAE） dopants coupled with UV-light modulation. With a 10-fold enhancement of the thermopower from 13.5 μV K-1to 135.4 μV K-1and almost unchanged electrical conductivity, the DAE-doped PEDOT:PSS thin film achieved an extremely high power factor of 521.28 μW m-1K-2from an original value of 6.78 μW m-1K-2. The thermopower was positively correlated with the UV-light intensity but decreased with increasing temperature, indicating resonant coupling between the planar closed DAE molecule and PEDOT. Both the experiments and theoretical calculations consistently confirmed the formation of an interface state due to this resonant coupling. Interfacial entropy engineering of polarons could play a critical role in enhancing the thermoelectric performance of the organic film.

关键词： thermoelectric PEDOT: PSS interfacial occupied entropy resonant coupling UV-light modulation

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Orbital doublet driven even-spin Chern insulators

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Physical Review B 2024年第3期110卷 035161-035161页

quantum spin Hall insulators hosting edge spin currents hold great potential for low-power spintronic devices. In this paper, we present a universal approach to achieve a high and near-quantized spin Hall conductance plateau within a sizable bulk gap. Using a nonmagnetic four-band model Hamiltonian, we demonstrate that an even-spin Chern (ESC) insulator can be accessed by tuning the sign of spin-orbit coupling (SOC) within a crystal symmetry-enforced orbital doublet. With the assistance of a high-spin Chern number of CS=−2 and spin U(1) quasisymmetry, this orbital doublet driven ESC phase is endowed with the near double quantized spin Hall conductance. We identify 12 crystallographic point groups supporting such a sign-tunable SOC. Furthermore, we apply our theory to realistic examples, and show the phase transition from a trivial insulator governed by positive SOC in the RuI3 monolayer to an ESC insulator dominated by negative SOC in the RuBr3 monolayer. This orbital doublet driven ESC insulator, RuBr3, showcases nontrivial characteristics including helical edge states, near double quantized spin Hall conductance, and robust corner states. Our work provides different pathways in the pursuit of the long-sought quantum spin Hall insulators.

关键词： Chern insulators quantum spin Hall effect 2-dimensional systems Topological materials First-principles calculations Group theory

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Local Excitation of Kagome Spin Ice Magnetism Seen by Scanning Tunneling Microscopy

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Physical Review Letters 2024年第4期133卷 046503-046503页

作者： Hanbin Deng Tianyu Yang Guowei Liu Lu Liu Lingxiao Zhao Wu Wang Tiantian Li Wei Song Titus Neupert Xiang-Rui Liu Jifeng Shao Y. Y. Zhao Nan Xu Hao Deng Li Huang Yue Zhao Liyuan Zhang Jia-Wei Mei Liusuo Wu Jiaqing He Qihang Liu Chang Liu Jia-Xin Yin Department of Physics Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area (Guangdong) Shenzhen China Shenzhen Key Laboratory of Advanced Quantum Functional Materials and Devices Shenzhen Institute for Quantum Science and Engineering Institute for Advanced Studies School of Physical Science and Technology Guangdong Provincial Key Laboratory of Computational Science and Material Design

The kagome spin ice can host frustrated magnetic excitations by flipping its local spin. Under an inelastic tunneling condition, the tip in a scanning tunneling microscope can flip the local spin, and we apply this technique to kagome metal HoAgGe with a long-range ordered spin ice ground state. Away from defects, we discover a pair of pronounced dips in the local tunneling spectrum at symmetrical bias voltages with negative intensity values, serving as a striking inelastic tunneling signal. This signal disappears above the spin ice formation temperature and has a dependence on the magnetic fields, demonstrating its intimate relation with the spin ice magnetism. We provide a two-level spin-flip model to explain the tunneling dips considering the spin ice magnetism under spin-orbit coupling. Our results uncover a local emergent excitation of spin ice magnetism in a kagome metal, suggesting that local electrical field induced spin flip climbs over a barrier caused by spin-orbital locking.

关键词： Electrical properties Magnetism Spin ice Scanning tunneling microscopy Scanning tunneling spectroscopy Two-level models

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Pressure-induced phase transition in transition metal trifluorides

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Chinese Physics B 2022年第10期31卷 102-106页

作者： Peng Liu Meiling Xu Jian Lv Pengyue Gao Chengxi Huang Yinwei Li Jianyun Wang Yanchao Wang Mi Zhou State Key Laboratory of Superhard Materials&International Center for Computational Method and Software College of PhysicsJilin UniversityChangchun 130012China Laboratory of Quantum Functional Materials Design and Application School of Physics and Electronic EngineeringJiangsu Normal UniversityXuzhou 221116China MIIT Key Laboratory of Semiconductor Microstructure and Quantum Sensing Nanjing University of Science and TechnologyNanjing 210094China

As a fundamental thermodynamic variable, pressure can alter the bonding patterns and drive phase transitions leading to the creation of new high-pressure phases with exotic properties that are inaccessible at ambient pressure. Using the swarm intelligence structural prediction method, the phase transition of TiF_(3), from R-3c to the Pnma phase, was predicted at high pressure, accompanied by the destruction of TiF_6 octahedra and formation of TiF_8 square antiprismatic units. The Pnma phase of TiF_(3), formed using the laser-heated diamond-anvil-cell technique was confirmed via high-pressure x-ray diffraction experiments. Furthermore, the in situ electrical measurements indicate that the newly found Pnma phase has a semiconducting character, which is also consistent with the electronic band structure calculations. Finally, it was shown that this pressure-induced phase transition is a general phenomenon in ScF_(3), VF_(3), CrF_(3), and MnF_(3), offering valuable insights into the high-pressure phases of transition metal trifluorides.

关键词： high-pressure structure transition crystal structure prediction high-pressure x-ray diffraction experiments transition metal

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